1. Field of the Invention
The present invention relates to determining the locations of mobile stations, more particularly, to a method and system for enabling a wireless signal that a mobile station receives via a repeater to be used for location determination.
2. Description of Related Art
The Global Positioning System (GPS) is often used to determine the locations of mobile stations, for example, to provide location-based services. Such location-based services include services, such as roadside assistance, direction assistance, vehicle tracking, weather forecasts, etc., that make use of the locations of mobile stations. Although GPS-based positioning methods are often the most accurate type of positioning method, a mobile station may not always be able to receive good signals from a sufficient number of GPS satellites. This can occur, for example, because of poor weather conditions or because the mobile station is in an urban or indoor environment.
Because of the limitations of GPS-based positioning methods, terrestrial positioning methods have also been developed. In terrestrial positioning, signals from one or more terrestrial locations, such as signals transmitted by base transceiver stations (BTSs) in wireless telecommunications networks, are used to determine the location of a mobile station. Such terrestrial positioning methods include advanced forward link trilateration (AFLT) methods, hybrid methods, and cell ID methods.
In a typical AFLT method, a mobile station measures the phases of pilot signals transmitted from four BTSs. In CDMA networks, each pilot signal is synchronized to a standard time, such as a GPS time (although each BTS may be associated with a particular time offset). As a result, the phase of a pilot signal from a BTS that is measured by a mobile station represents the transmission delay of the pilot signal, which, in turn, can be used to calculate the distance between the mobile station and the BTS. In this way, the distances between the mobile station and the four BTSs may be calculated. These four distance measurements can then be used to calculate the mobile station's location and time offset, provided the locations of the BTSs and their respective time offsets are known.
A mobile station's location may also be calculated using a hybrid method. In a hybrid method, measurements of one or more terrestrial signals are combined with measurements of signals from one or more GPS satellites to determine a mobile station's location. For example, one, two, or three of the distance measurements that would be used for AFLT may be replaced by measurements of signals from GPS satellites.
Cell ID methods are typically simpler but less accurate than AFLT and hybrid methods. The wireless coverage area (e.g., the cell or sector) currently serving the mobile station is identified, and the mobile station's location is taken to be the center point of the wireless coverage area. An uncertainty in the mobile station's location may also be provided. The uncertainty may roughly correspond to the radius of the wireless coverage area.
To support AFLT, hybrid, and cell ID methods, CDMA networks often store parameters used for location determination in a base station almanac (BSA). These location-determination parameters may include: (i) the locations (e.g., latitude and longitude coordinates) of the BTSs; (ii) forward link calibration (FLC) values that may be used to correct for the time offsets of the BTSs; (iii) and cell/sector center points and uncertainties. During a location determination session, a network element such as a location engine may calculate the mobile station's location based on signal measurements reported by the mobile station (e.g., phase measurements of pilot signals from one or more BTSs) and the location-determination parameters stored in the BSA.
However, repeaters are increasingly being used in wireless telecommunication network to expand wireless coverage. For example, repeaters are often used to fill coverage “holes” in areas such as office buildings, highways, and tunnels. On the downlink, a repeater typically receives signals from a donor BTS (either wirelessly or via a cable), amplifies the signals, and repeats the signals into a repeater coverage area. On the uplink, a repeater typically receives wireless signals from mobile stations operating in the repeater coverage area, amplifies the signals, and forwards the signals to the donor BTS. A repeater typically amplifies the uplink and downlink signals without changing the content of the signals.
Although repeaters are often effective at improving wireless coverage, repeaters can cause difficulties for terrestrial positioning methods. As noted above, AFLT and hybrid methods calculate distances based on the transmission delays (measured as phases) of wireless signals transmitted by one or more BTS. However, repeaters impose additional transmission delays on the signals that they repeat. As a result, when a mobile station receives a wireless signals via a repeater, the distance to the BTS that is calculated from the signal's transmission delay may not be accurate, even when corrected by the BTSs FLC value.
Cell ID methods can also be affected by repeaters. Because a repeater is typically used to expand wireless coverage beyond what the donor BTS provides directly, the donor BTS's cell/sector center point may not be a useful estimate of the location of a mobile station that is being served by the repeater.
Current approaches for addressing location-determination problems caused by repeaters have significant disadvantages. In an approach that has been used for AFLT and hybrid methods, the BSA flags each BTS that has a repeater. Signals from the flagged BTSs are then excluded from AFLT and hybrid location determinations. However, if one or more of the wireless signals received by a mobile station for location determination is excluded as being from a flagged BTS, then the number of available wireless signals may be insufficient to obtain a valid AFLT or hybrid location fix. The less-accurate cell ID method might then be used as a fall-back.
In an approach that has been used for cell ID methods, the total coverage area provided by the donor BTS and the repeater is used to calculate the cell/sector center point. However, increasing the size of the coverage area used for the calculation will also increase the uncertainty that is associated with using the center point as the mobile station's location.
Accordingly, there is a need to provide ways of using wireless signals that are received via repeaters for location determination.